1. Field of the Invention
The present invention relates to a back light unit used for a liquid crystal display device (LCD) and more specifically, the present invention relates to a light guide panel of the back light unit for an LCD which is used for a display unit of a computer system, AV(Audio and Video) system or mobile telephone system.
2. Description of the Background Art
Generally, a liquid crystal display device includes a liquid crystal panel 10 which has two substrates and a liquid crystal inserted therebetween, a back light unit 20 which is disposed under the liquid crystal panel 10 and cases 1 and 2 as shown in FIG. 1 which is a perspective view showing the structure of the conventional LCD.
The back light unit 20 is arranged to distribute a light from a light source uniformly over the surface of the liquid crystal panel 10. There are a few kinds of back light units 20 such as a direct back light type (or direct type) and an edge light type.
Referring to the FIG. 2 which is a cross-sectional view of the back light unit 20 cut along line II--II of FIG. 1, the direct type back light unit 21 includes a housing 70 which has a reflective sheet 60 disposed at the bottom and a lamp 50 such as a fluorescence cathode tube at the bottom portion of the housing 70, a diffusing sheet 40 in the housing 70 and a light condenser 30 disposed on the diffusing sheet 40. Because the lamp 50 is disposed inside the display area of the liquid crystal panel 10, the brightness distribution of the liquid crystal panel is not uniform and the liquid crystal and the TFT switching elements can be damaged by the heat energy of the back light source.
Therefore, some distance or space between the lamp 50 and the diffusing sheet 40 is required. A patterned light blocking element disposed at the edge of the lamp 50 may also be needed. Because of these required structural elements, it is very difficult to reduce the thickness of the direct type back light unit 21.
Referring to the FIG. 3 which is a cross-sectional view of the back light unit 20 cut along line III--III of FIG. 1, the edge light type back light unit 22 includes a light guide 80, a lamp 50 which is attached to at least one edge of the light guide 80, and a U-type reflector 61 which surrounds the lamp 50. An open portion of the reflector 61 is fixed at the edge of the light guide 80, a reflecting sheet 60 at the bottom of the light guide 80, a diffusing sheet 40 on the light guide 80 and a light condenser 30 on the diffusing sheet 40.
Because the lamp 50 is disposed at the edge of the light guide 80, the thickness of the LCD can be minimized. Furthermore, the liquid crystal and the TFT switching elements are not damaged by the heat energy of the back light. Therefore, the edge light type has some advantages over the direct type in that it is thinner and provides enhanced picture quality.
FIG. 4 is a cross-sectional view which illustrates a conventional structure of the edge type back light unit. The edge type back light unit includes a light guide 80 which has a printed dot pattern or a carved-out V-shape pattern on at least one surface, a diffusing sheet 40 disposed on the light guide 80 which is made of half-transparent PET or polycarbonate, a lamp 50 which is attached to at least one side of the light guide 80 and a light condenser 30 disposed on the diffusing sheet 40. Because the diffusing sheet 40 is half-transparent, the sheet 40 can hide the printed dot pattern or the carved-out V-shape pattern of the light guide 80. The light condenser 30 gathers diffused light from the diffusing sheet 40. For example, as shown in FIG. 4, if the incident light has an angle (.theta.)1 and (.theta.)2 relative to the normal line A of the surface of the light condenser 30, then the light transmitted through the light condenser 30 is refracted at the transmittance surface of the light condenser 30. Preferably, two of the light condensers 30 can be used for enhancing the brightness.
However, the brightness of the liquid crystal panel 10 decreases according to the distance from the lamp 50. In order to overcome this problem in the edge light type, some solutions have been attempted. One solution is to form the light guide to have a tapered shape wherein the thickness of the edge of the light guide 80 which contacts the lamp 50 is relatively thick and the counter edge is relatively thin. Another solution is to coat a plurality of dots on the surface of the light guide 80 wherein the area of the dots is varied by the distance from the lamp 50.
Referring to FIGS. 5-9 which are some examples of the edge type back light units, the structure of the light guide 80 will be explained in detail. FIG. 5 is one example of the device shown in FIG. 4. FIGS. 6(a) and 6(b) are cross-sectional views which illustrate the light guide shown in FIGS. 4 and 5. FIGS. 7, 8 and 9 illustrate variable patterns formed on one side of the light guide shown in FIGS. 4 and 5.
The light guide 80 is generally made of a transparent material such as an acrylic resin. In addition, a dot pattern 100 is printed as shown in FIGS. 6(a) and 9 or a V-shape pattern 105 is carved as shown in FIGS. 6b and 7. By forming these patterns, the brightness of the light which is incident to the liquid crystal panel is made uniform. In the case of the dot pattern, the size of the dots increases according to the distance from the lamp 50 attached on one side of the light guide 80 as shown in FIG. 6(a). In the case of the carved V-shaped pattern, the distance between adjacent patterns decreases according to the distance from the lamp 50 as shown in FIG. 6(b). The light guide 80 can be a plate type or a tapered type.
The conventional edge light type back light unit has some problems as described hereinafter.
First, in the case of the dot pattern 100 (shown in FIG. 9) which is located at the bottom side 90 of the light guide 80, the printed material is one of a volatile hardening ink and a UV hardening ink which includes titan oxide particles for diffused reflection of the light. It is very difficult to control the viscosity of the printed material when the ink is printed via a screen printing method because the size of the dots 100 is variable according to the distance from the lamp 50.
For example, the dots 100 for diffusing reflection of the back light have various sizes from tens of .mu.m to thousands of .mu.m. When the dots having a size in the range of tens of .mu.m are printed, the viscosity of the ink should be lower because the ink can hardly pass through the pattern of the screen. However, when the dots having a size of thousands of .mu.m are printed, the viscosity of the ink should be larger because the ink can easily pass through the pattern of the screen. Therefore, if the viscosity of the ink has a high value, some of small dots can not be printed. On the other hand, if the viscosity of the ink has a low value, some of large dots can be blotted. Furthermore, the liquid crystal can be damaged by the heat energy of the light at the dots so that the quality of the LCD is inferior.
Second, in the case of the carved V-shape pattern 105 (shown in FIGS. 7 and 8) which is located at the bottom side 90 of the light guide 80, it is very difficult to determine the optimum condition of the distance variation of the patterns. In addition, determination of the maximum status of the brightness distribution requires a great deal of time, effort and expense.